Auroral asymmetries in the conjugate hemispheres
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(and where KuaFu B can do better……)
Nikolai Østgaard
University of Bergen, Norway
Coauthors:
B. Krøvel Humberset, K.M. Laundal, A.Aasnes, S. Haaland
H. U. Frey, J. B. Sigwarth, J. Weygand
1
August 2011
Outline:
1.
Asymmetric auroral intensities
2.
Substorm onset location asymmetries
3.
What happen to asymmetry during substorm expansion phase
4.
Theta Aurora in only one hemisphere
5.
Cusp Aurora in both hemispheres
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August 2011
1. Very asymmetric
aurora –
closed field
lines
Normal
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Movie:
2139-2153 UT
WIC/FUV - IMAGE
VIS Earth Camera - Polar
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August 2011
2. Asymmetric auroral intensities
intensities
Normal text - click-- Asymmetric
to
edit
Poleward expansion
Conditions:
-Seasons:
-North-summer
-South- winter
-Bx dominated IMF
-By ~0
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August 2011
Interpretation
Laundal and Østgaard, Nature, 2009
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x
X
A combination of
- the more efficient solar wind dynamo
- inter-hemispheric currents:
- Persistent spot in south (dynamo + interhemispheric
currents)
- Transient spot in the north (interhemispheric
currents) and the significant increase in reconnection
(poleward expansion) can explain its transient
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August 2011
character.
2. Asymmetric substorm onset location
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Westward bulge: SH ~1.1 MLT
duskward of NH
Duskward – BY negative !
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August 2011
Statistical distribution – Polar and IMAGE
•
•
•
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6600 substorms by IMAGE and Polar
4671 Northern hemisphere
1930 Southern hemisphere
Time shifts ACE and Wind:
-10 Re +/- 5 min average
-20 Re
+/- 5 min average
-10 Re - 40 min average
294/108
640/293
696/264
1117/460
1121/433
803/372
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August 2011
Statistical distribution
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August 2011
Average onset location – clock angle
-10 Re
-20 Re
-10 Re
+/- 5 min average
+/- 5 min average
- 40 min average
Clock text
angle controls
averageto edit
Normal
click
onset location in each
hemispheres
Onset locations are still
distributed in a large range of
MLTs
Interhemispheric asymmetry
might be more robust result
Indirect indication that tail
reconnection is significant for
Northward IMF
MLT  0.53 sin(C  4.8)  0.17
August 2011
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Average onset location – By, IMF to -10 Re
Reveals saturation towards dusk for
- positive By in north
- negative By in south
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By > 0
A)
B)
- - - ++
- - - - - ++ + ++
- - - - - - + + + + ++
- - - - - - - + + + + +++
- - - - - - - - + + + + +++
- - - - - - - - + + + + + + ++
- - - - - - - - + + + + + + ++
- - - ++
- - - - - ++ + ++
- - - - - - + + + + ++
- - - - - - - + + + + +++
- - - - - - - - + + + + +++
- - - - - - - - + + + + + + ++ y
- - - - - - - - + + + + + + ++
z
- - - ++
- - - - - ++ + ++
- - - - - - + + + + ++
- - - - - - - + + + + +++
- - - - - - - - + + + + +++
- - - - - - - - + + + + + + ++
- - - - - - - - + + + + + + ++
- - - ++
- - - - - ++ + ++
- - - - - - + + + + ++
- - - - - - - + + + + +++
- - - - - - - - + + + + +++
- - - - - - - - + + + + ++ ++
- - - - - - - - + + + + + + ++
C)
By < 0
D)
By
20 x
= -18 RE
GSM
xGSM = -18RE
+1
[RE]
10
zGSM
Asymmetric
penetration of
IMF By in the
deep tail
(Khurana et al.,
1996) extends to
closed field lines
z
0
y
-10
-20
20
-1
10
0
yGSM [RE]
-10
-20
20
MLT  0.88  sin(
August 2011
10
0
yGSM [RE]
-10
-20
BY
 9.3)
12 nT
10
Østgaard et al, GRL, L08104, 2011.
3. What happen to asymmetries during expansion phase
IMAGE
Oct 22, 2001:
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5 hours of
conjugate images
2 substorms
VIS Earth
August 2011
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Method
1.
2.
3.
4.
5.
Used available DMSP data to ensure pointing accuracy
Visual Inspection method
Mapped onto rectangular magnetic grid -0.1 MLT - 1° Maglat resolution
1D correlation (cross correlation)
2D correlation (chi square)
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August 2011
Substorm assymmetry
Grey diamonds:
all images < 1 min overlap
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Black diamonds:
- time overlap (< 21 sec)
- corr.coeff > 0.4 (1D)
- chi square < 0.4 (2D)
• Onset is asymmetric
• During expansion phase the
asymmetry disappears –
regardless of clock angle or
By.
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August 2011
Rectifying the field lines during expansion phase
i) Asymmetry is created in the mid-tail
ii) I inverted-V structures are formed which decouple
magnetosphere from the ionosphere,–
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iii) and magnetic stress is released (Haerendel, 2007
rectifying the asymmetric field lines
b) Twist on field lines create net electric field pointing
from north to south
c) Faradays loop on closed field lines
 E  u  Bdl  
∆
us  u n 
d
B  dA  0
dt
E  vB  0
E|| l
BL
Østgaard et al, GRL, L03101,2011
August 2011
14
3. Theta
4. Cusp aurora
Non-conjugate theta (transpolar
arcs) consistent with IMF Bx
but more observations to see if
this holds statistically
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Østgaard et al., Geophys. Res. Lett., 2003
Also the cusp aurora in both
hemispheres
IMF By: longitudinal shift
Tilt angle: latitudinal shift
Østgaard et al., Geophys. Res. Lett., 2005
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August 2011
Summary:
1. Asymmetric auroral intensities
•
Indication of interhemispheric currents
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2. Substorm onset asymmetries
•
Usually asymmetric locations at substorm onset
•
Strong statistical correlation with clock angle and By
3. Substorm expansion phase
•
Field lines are rectified during expansion phase
4. Theta Aurora
•
Maybe a IMF-Bx control of theta aurora?
5. Cusp Aurora
•
IMF control of longitudinal shifts and maybe a tilt angle of the
latitudinal shift
?
KuaFu can address these questions and many more ….
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August 2011
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Thank you!
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August 2011
Tilt angle effects
Onset in south is downward
of north onset for positive tilt,
i.e., south=winter
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More intense nightside FACs were found by Ohtani et al., 2005
in the winter hemisphere
Needs more data to confirm!
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August 2011
Summary and challenges:
Cusp and reconnection – temporal behavior
1. We have shown the asymmetry of cusp location
2. Challenge: To determine the relative reconnection rate
1. Optical measurements: Relative intensity of the cusp spot
2. Even better: Reconnection rate with radars (Pinnock et al., 1999) and
optical data. More than one radar, get the relative reconnection rate.
Critical: OC boundary position, tilt and speed.
3. Relative size of the polar caps, optics and radars to get the opening and
closing of magnetic flux in the two hemispheres
Transpolar arcs and theta aurora
1. We have shown that theta can be non-conjugate, but to determine what
controls it; more observations are needed: ground based optics, GUVI and
SSUSI, low altitude satellites.
Subtsorm onset location
1. The IMF clock angle control is well documented. The global asymmetry
suggests that the tilt effect and difference in FAC may be insignificant.
Dynamic features of the expansion phase of substorms
1. The potential mismatch??
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August 2011
How IMF affect the magnetospheric
configuration
The penetration of the IMF field
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Has been directly observed in the outer
magnetosphere by Sibeck et al,
1985 and the inner magnetosphere
Wing et al., 1995
noon
midnight
dawn
Wing et al, 1995 20
August 2011
Faraday’s law
Reconnection rate - Method –
Vasyliunas [1984]
 E  u  Bdl  
d
B  dA
dt
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Assumptions
E  vB  0
(>300 km)
E||  0
Em  m  v  ui  Bi  i  Ei  i
E
m
40RE

 i
i  cosRE
Em
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August 2011
Is it real?
1) Relative variations in counts
North-WIC: red
South-VIS: black
2) Electron Energies and different cameras:
Dawn-spot:
10 kR (WIC)
~5 kR (VIS)
High electron energies will give higher
intensity in WIC than in VIS,
If 25 keV mean energy -> 40 mW/m2 and
should give about 10 kR in VIS, but we see
only 5 kR.
Energy cannot explain the dusk differences.
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3) N2/O2 ratio increases due to heating WIC
would be slightly brighter – this effect is very
small; intensity differences would be
underestimated at dusk and exagerated at
dawn.
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August 2011
Challenge: Dayside reconnection rate with radars and
imaging
Pinnock et al., 1999
Two critical points:
1) get the OC boundary and its speed correct Imaging is needed
2) More than one radar: Differences in E_rec strength are
interpreted as moving away from the throat region (i.e. antiparallel site)
Vasyliunas [1984]
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IMF Bz<0
Variable By
North
South
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August 2011